1. Field of the Invention
The present invention is directed to heat exchanger elements of the type formed by two layers of thin-film plastic material, in which flow passages are formed by the advantageous placement of seams or welds between the two layers, for example, by heat sealing or ultrasonic welding, to produce a serpentine passage through the heat exchanger element. The present invention is further directed to such heat exchanger elements, as may be employed in the field of absorption heat pump apparatus.
2. Prior Art
Gas-fired absorption heat pump apparatus, particularly absorption chillers, in order to have high enough coefficients of performance, in order to be efficient and competitive with presently used, more conventional cooling apparatus, require the use of large area heat exchanger elements, in the absorber and evaporator sections of the heat pump systems. However, with increased size, comes increased weight and cost. Accordingly, alternatives to metal-construction heat exchanger elements are being sought.
One alternative which is being explored, is the use of plastic film heat exchanger elements, in which each of the one or more elements is composed of two sheets of thin plastic film, which are bound or welded to one another, with serpentine passages formed between the sheets by a plurality of seams or welds formed by thermal or ultrasonic welds.
Such heat exchanger elements typically will be situated in a working environment of less than atmospheric pressure, in a vacuum vessel and during operation, are exposed to a continuous partial vacuum. Accordingly, the material from which the heat exchanger element is constructed must be resistant to creep, particularly since such heat exchanger elements are intended to be long lifespan components (10-20 years).
An additional consideration is that the plastic material must be resistant to the migration through the material of non-condensable gases, which may be entrained in the fluid (typically water, or a water-based solution). If such gases escape through the heat exchanger walls into the vacuum vessel, then the efficiency of the absorber/evaporator is diminished. Currently known metallic heat exchanger element based systems have scheduled purges of such gases from their vacuum vessels at regular intervals. Accordingly, it would be desirable to construct a plastic film heat exchanger element which has a low enough permeability such that purges would not be required more often than with conventionally known systems.
A plastic-film heat exchanger element is disclosed in May, U.S. Pat. No. 4,933,046, in which the heat exchanger element is used as the condenser for a water purifying system. The heat exchanger element has passageways, formed by thermal welds, which are substantially rectangular in plan, and generally circular in cross-section, with cross-sectional areas which decrease, proceeding from an upper (steam) portion to a lower (condensate) portion.
Tubin, U.S. Pat. No. 4,118,946, discloses a personnel cooler, featuring a plastic film heat exchanger element configured as a vest, in which cooling fluids are disposed.
Dodds, U.S. Pat. No. 4,693,302, discloses a thin plastic sheet heat exchanger element having serpentine fluid passages formed by curvilinear patterns of welds. One embodiment is made of two films of equal thickness, resulting in fluid passages that are substantially round in shape. In another embodiment, sheets of unequal thickness are employed, which yield fluid passages which are flattened on one side.
Plastic film heat exchanger elements cannot operate at the same kinds of pressures that metal-constructed heat exchanger elements can. Accordingly, it is desirable to increase the potential operating pressure of such heat exchanger elements. The bends in the serpentine passageways are generally those portions of the heat exchanger elements which are most vulnerable to failure.
Furthermore, since additional sheet material area is necessary to accommodate the fluid passages, during manufacture, it is difficult to avoid the generation of creases or folds, in the corners of the bends. Such creases create flow inefficiencies, and create sites for fatigue and potential failure, as well.
Accordingly, it would be desirable to provide a plastic film heat exchanger element which would be substantially free of such creases or folds in the corner regions.